This invention relates to an AC-DC or DC-AC electric power converting station (hereinbelow called DC converting station) in a DC transmission system, and more particularly to sealed DC converting station in which each constructional unit or element is arranged so as to be conveniently maintained and inspected or exchanged in case of a fault.
Recently, a DC power transmitting system having many characteristic features superior to an AC power transmitting system has been put into practical use. In the DC power transmitting system, indoor-type, air-insulated and air-cooled controlled semiconductor switching elements, for example, thyristors, are utilized as one of the most important units of a power converting station, with the thyristors and the controlling units thereof being installed indoors in many instances. In the meantime, oil-cooled and oil-insulated-type thyristors have been developed and used commercially in accordance with the requirements for units having large capacities and operating at high voltages. However, units and elements in a DC converting station, such as thyristors, DC transformers, DC potential transformers, reactors, arresters, and disconnecting switches, are connected to each other through bus bars supported by insulators in the air or through bushings.
On the other hand, a compact power converting station sealed in an air tight manner and insulated with sulfur hexa-fluoride (SF.sub.6) gas has been built for use as a converting station in an AC power transmitting system to overcome problems such as air pollution, public hazards, earthquakes and the lack of a sufficiently large site to accommodate the station. For many of the same reasons, SF.sub.6 gas insulation is also strongly desired in DC converting stations for insulating the air-insulated parts of bushings and supporting insulators. Particularly, as to air pollution, since the DC flashover voltage (a) under contaminated conditions is lower than the AC flash over voltage (b) under similar contaminated condition, as shown in FIG. 1, the problem of air pollution determines that whether DC power transmission is practicable or not.
In the AC converting station, the constructional units or elements are arranged about circuit interrupters which are the most important elements for the station and need periodical inspection and maintenance. For this reason, the circuit interruptors are constructed to be compact. Likewise, in the DC converting station too, it is important to arrange units or elements by taking into consideration the location of the thyristors which are the most important units in this station, requiring periodic inspection and mantenance. In the DC converting station, since DC bus bars and AC three phase bus bars are used and since a plurality of thyristors are connected in series to obtain a high voltage, the arrangement or location of these bus bars and units are naturally different from that of the AC converting station.